is there something special to use the guardian legion vendors in velike?

Comments

First one unlocks based on doing the basic Guardian Legion quest from Seir (Enemies Foreign and Domestic, IIRC), the next two based on item level (431 and 439) triggered quests. You get access to the vendor aspect upon the completion of each of the three quests.

The unlocks are character based, so you need to do that on every character you want to use them on.